It's not that complex. Just split your lines on tab characters, using split( /(\t)/, $line ). Those parens around \t make sure the tabs themself will be included in the elements of split, too. From there on, it's simple: print out every piece that's not a tab character, and keep track of how many characters you've printed so far. If you eventually do run into a tab character, you just print enough spaces to end up at a tab boundary. To make sure things keep running smoothly, you want to make sure that those spaces also count for the number of characters printed so far.

Well, even if split didn't had that handy bit of also returning the things it captured, you could've gone my @parts = $line =~ m/(\t|[^\t])/g;, which does essentially the same. With that regex you tell perl, "gimme an array of all tabs and all sequences of non-tabs."